UNIVERSITY  OF  CALIFORNIA         agricultural  Experiment  station 
:  of  agricul 
BERKELEY 


College  of  agriculture  benj.  ide  wheeler,  president 

THOMAS    FORSYTH    HUNT,    Dean  and  Directof 


CIRCULAR  No.  100 
(June,  1913) 


PRUNING   FROSTED   CITRUS  TREES 


BY 
J.    ELIOT  COIT 


It  has  previously  been  pointed  out  by  this  Station  that  it  is  wise 
to  defer  pruning  frost  injured  #  trees  until  it  is  perfectly  clear  just 
how  far  back  the  wood  has  been  killed.  After  the  line  of  demarcation 
between  living  and  dead  wood  becomes  apparent,  no  time  should  be 
lost  in  taking  the  necessary  steps  toward  reconstructing  the  top  in 
the  best  way  with  the  least  possible  loss  of  time. 

The  problems  involved  in  mending  and  reforming  the  tops  of 
frosted  citrus  trees  fall  naturally  into  two  classes,  viz..  those  having  to 
do  with  very  young  trees  and  those  concerned  with  large  mature  trees. 

REHEADING  YOUNG  TREES 

When  a  one  or  two-year-old  tree  has  been  killed  to  a  point  near 
the  ground  it  should  first  be  determined  in  each  individual  case 
whether  the  dead  wood  extends  below  the  bud  union.  If  there  remains 
enough  live  wood  above  the  union  to  grow  a  strong  shoot,  such  a 
shoot  may  be  allowed  to  grow  and  become  the  trunk  of  the  new  tree, 
all  other  shoots  being  suppressed.  If  on  the  other  hand  the  trunk  is 
killed  below  the  union  it  will  be  necessarj^  to  set  a  new  bud.  The 
opportunity  is  thus  presented  of  reconsidering  the  relative  profits  to 
be  derived  from  oranges,  lemons  or  pomelos.  In  case  the  root  is  sweet 
orange  the  bud  may  be  set  immediately,  provided  there  is  sufficient 
room  to  do  so  without  excavating  the  soil,  for  in  no  case  should  a  bud 
be  set  below  the  soil.  In  case  the  trees  have  been  killed  to  the  ground 
it  is  well  to  select  the  most  vigorous  sprout  and  bud  into  this  two  or 
three  inches  above  the  ground.    If  the  root  is  sour  orange  or  pomelo 


it  is  best  to  place  the  bud  six  or  eight  inches  from  the  ground  on  a 
strong  sprout.  This  will  result  in  a  somewhat  hardier  tree  and  one 
more  resistant  to  gum  disease. 

In  all  cases  where  a  new  trunk  must  be  grown  on  trees  already 
planted  in  orchard  form,  it  is  necessary  of  course  to  stake  the  trees 
and  tie  them  every  few  inches,  taking  care  to  disbud  often  and  remove 
all  sprouts  from  the  root,  eventually  heading  the  tree  at  the  desired 
height.  A  stake  larger  than  the  customary  building  lath  is  advisable 
in  order  to  prevent  injury  by  passing  teams  and  tillage  tools. 

Where  the  tops  of  young  trees  are  partly  frozen  it  will  be  found, 
especially  with  lemons,  that  the  more  rapid  growing  upright  shoots 
have  been  killed  while  the  fine  fruiting  brush  has  been  spared. 
Instead  of  cutting  back  all  of  these  injured  shoots  in  proportion  to 
the  injury,  it  is  much  better,  especially  with  lemons,  to  take  out  most 
of  the  very  strong  upright  canes  entirely.  If  all  are  merely  cut  back 
and  allowed  to  remain  each  one  will  throw  up  several  similar  shoots 
which  in  turn  may  be  cut  back,  the  final  result  being  a  broom-like 
growth  with  far  too  many  shoots.  The  proper  pinching  and  thinning 
of  such  a  multiplicity  of  shoots  will  involve  much  more  labor  than 
the  commercial  orchardist  can  afford.  It  is  better,  therefore,  to  remove 
most  of  the  upright  vegetative  shoots  entirely  and  rely  on  new  shoots 
from  the  horizontal  wood  to  enlarge  the  framework  of  the  top.  All 
water  sprouts  arising  from  the  trunk  and  around  the  crotches  should 
be  vigorously  suppressed.  It  is  a  good  plan  to  whitewash  the  exposed 
trunks  in  order  to  prevent  sunburning  of  the  bark. 

In  many  cases  the  bark  of  the  trunks  of  young  trees  has  been  split 
by  the  frost  and  has  dried  out  resulting  in  a  strip  of  dead  bark  on 
one  side,  usually  the  southwest  side.  This  condition  is  not  necessarily 
serious,  provided  there  remains  a  strip  of  live  healthy  bark  of  sufficient 
width  to  provide  a  flow  of  sap  to  the  top.  The  living  bark  will 
gradually  increase  in  width  and  extend  over  the  dead  areas,  in  time 
covering  them  entirety.  In  order  to  prevent  the  entrance  of  wood 
decaying  fungi  it  is  well  to  paint  the  dead  areas  with  white  lead  or 
asphalt  paint. 

BEHEADING  OF  MATURE  TREES 

Orange  trees  are  much  more  resistant  to  frost  than  lemon  trees 
Over  certain  large  areas  the  recent  freeze  has  ruined  the  fruit  and 
completely  defoliated  the  trees  without  killing  back  the  wood  to  any 
appreciable  extent.  In  such  a  case  the  trees  need  no  extra  pruning, 
although  while   the  leaves  are   absent   a   good   opportunity   presents 


itself  to  remove  conflicting  branches  and  thin  the  brush  with  greater 
ease  and  efficiency  than  is  possible  when  the  trees  are  covered  with 
leaves. 

Where  the  wood  is  killed  back  to  any  extent,  however,  too  many 
shoots  are  apt  to  start  from  the  live  wood  and  many  suckers  from 
about  the  crotches.  Theoretically  the  shoots  should  be  thinned  out, 
leaving  only  what  is  needed.  Those  left  will  be  stronger  and  bear 
more  fruit  than  if  such  a  large  number  were  allowed  to  struggle  for 
life.  Practically,  however,  such  a  course  would  require  more  labor 
than  is  available  and  large  growers  will  have  to  adopt  the  next  best 
course,  viz.,  that  of  cutting  out  all  the  dead  wood  and  thinning  the 
new  growth  by  taking  out  bodily  a  certain  number  of  branches. 

Where  lemon  trees  have  been  killed  back  severely  it  is  best  to 
withhold  or  decrease  the  fertilizer  during  the  succeeding  year  in  order 
that  a  more  moderate  growth  may  take  place.  Frozen  trees  have  been 
thrown  out  of  balance.  They  have  the  same  amount  of  root  as  before 
which  is  now  pouring  sap  into  a  much  reduced  top.  The  result  is 
bound  to  be  a  tremendous  new  growth  breaking  out  all  over  the  trunk 
and  branches.  At  best  a  great  deal  of  labor  wrill  be  required  to 
properly  sucker  the  trees.  If  additional  stimulation  is  brought  about 
by  the  application  of  quick  acting  fertilizers  this  trouble  will  be 
increased.  Wise  lemon  growers  prefer  to  build  the  new  head  out  of 
the  more  moderate  growth  of  fruiting  wood  rather  than  out  of  water 
sprouts.  It  is  not  necessary  or  always  advisable  to  root-prune  frosted 
trees,  but  if  hard-pan  or  plow-sole  exists  it  may  be  broken  up  with  a 
subsoil  plow  at  this  time  with  less  injury  to  the  trees  from  cutting  large 
roots.  The  number  of  roots  which  may  be  cut  with  impunity  will  of 
course  depend  upon  the  amount  the  tops  of  the  trees  have  been  killed 
back. 

Whenever  large  limbs  are  taken  out  it  is  well  to  apply  some  good 
antiseptic  to  the  cut  surfaces  in  order  to  prevent  the  entrance  of 
decay  organisms  and  to  prevent  undue  drying  out  and  checking  of 
the  wood.  White  lead  paint  and  asphalt  paint  are  good  dressings,  while 
liquid  grafting  wax  is  often  used.  A  good  formula  for  the  wax  is  6  lbs. 
resin,  1  lb.  beeswax,  and  1  pint  of  linseed  oil.  Another  wound  dressing 
which  is  very  cheap  and  has  been  highly  recommended  consists  of 
three  parts  melted  resin  mixed  with  one  part  warm  crude  petroleum. 

"It  seems  reasonably  certain  that  no  injury  to  the  tree  can  result 
from  any  of  the  materials  passing  from  the  frozen  oranges  back  into 
the  tree.  An  examination  of  those  oranges  which  have  been  too  badly 
frozen  to  be  fit  for  shipment,  shows  that  most  of  them  have  been  only 


partially  killed;  consequently  they  are  presumably  respiring  carbon 
dioxide.  This  loss  of  energy  would  be  saved  if  the  oranges  were 
removed  from  the  tree  rather  than  allowed  to  hang  all  summer. 
There  is  no  experimental  evidence,  however,  to  show  whether  this  loss 
is  sufficient  to  warrant  the  cost  of  early  removal  by  hand."  In  case 
the  fruit  is  picked  off  the  trees  it  may  safely  be  plowed  under,  thus 
adding  humus  and  some  plant  food  to  the  soil.  There  is  no  danger  of 
the  soil  becoming  acid  as  a  result  of  this  practice. 


